The present invention relates to hydraulic pistons. More specifically, the present invention relates to position sensors used to sense the relative position between a piston and a hydraulic cylinder.
Various types of displacement sensors are used to measure the relative position of a piston in a hydraulic cylinder. However, devices to remotely measure absolute displacement in harsh environments with a high degree of reliability are presently complex and costly. Examples of presently used technologies are Magnitostrictive devices that use time of flight of a mechanical signal along a pair of fine wires encased in a sealed metal tube, which is reflected back from a magnitostrictively induced change in the rod""s mechanical properties. Another technology uses an absolute rotary encoder, which is a device that senses rotation. The translational to rotary conversion is typically done with gears, or a cable or tape that is uncoiled from a spring loaded drum. Absolute encoders tend to suffer from limited range and/or resolution. Harsh environments that include high levels of vibration tend to exclude absolute etched glass scales from consideration due to their critical alignment requirements, their susceptibility to brittle fracture and intolerance to fogging and dirt. This technology also needs to be re-zeroed frequently.
Inferred displacement measurements such as calculating the translation of a cylinder by integrating a volumetric flow rate into the cylinder over time suffer from several difficulties. First, these devices are incremental and require frequent, manual re-zeroing. Secondly, they tend to be sensitive to environmental effects, such as temperature and density. They require measuring these variables to provide an accurate displacement measurement. Further, integrating flow to determine displacement tends to decrease the accuracy of measurement. This technology also is limited by the dynamic sensing range of the flow measurement. Flows above and below this range are susceptible to very high errors.
One technique used to measure piston position uses electromagnetic bursts and is described in U.S. Pat. Nos. 5,977,778, 6,142,059 and WO 98/23867. However, this technique is prone to emitting radiation into the environment and is difficult to calibrate.
An apparatus to measure relative position of a hydraulic piston in a cylinder, includes a rod extending along the direction of movement of the piston and the rod which is fixedly coupled to one of the piston or cylinder. The rod is configured to carry a microwave pulse. A sliding member is slidably coupled to the rod and fixedly coupled to the other of one of the piston or cylinder. The sliding member is configured to cause a partial reflection of the microwave pulse. The end of the distal rod also provides a reflection. Piston position is calculated as a function of reflected microwave pulses from the sliding member and the rod end.